Thermodynamics Lecture Notes: Laws of Thermodynamics, Heat Engines, and Entropy - Prof. Ta, Study notes of Physics

Download Thermodynamics Lecture Notes: Laws of Thermodynamics, Heat Engines, and Entropy - Prof. Ta and more Study notes Physics in PDF only on Docsity! 04/16/2003 Physics 103, Spring 2005, U. Wisconsin 1 Physics 103: Lecture 23 Thermodynamics, part 2 Laws of thermodynamics First: U = Q + W Second: Efficiency < 100% Engines and refrigerators Entropy and disorder 04/16/2003 Physics 103, Spring 2005, U. Wisconsin 2 Heat Engine For a cyclical process, U = 0 Its initial and final internal energies are the same Therefore, Qnet = Weng The work done by the engine equals the net energy absorbed by the engine The work is equal to the area enclosed by the curve of the PV diagram 04/16/2003 Physics 103, Spring 2005, U. Wisconsin 3 0 Q = U - W QH - QC QH - QC = -W = Weng for a heat engine TH TC QH QC Weng HEAT ENGINE TH TC QH QC Wref REFRIGERATOR system System of interest 0 Q = U - W QC - QH QH - QC = + W = Wref for refrigerator 04/16/2003 Physics 103, Spring 2005, U. Wisconsin 4 Lecture 23, Preflight 1,2 49% 32% 19% 0% 10% 20% 30% 40% 50% Consider a hypothetical refrigerator that takes 1000 J of heat from a cold reservoir at 100K and ejects 1200 J of heat to a hot reservoir at 300K. How much work does the refrigerator do? 1. 1200 J 2. 1000 J 3. 200 J correct W = Qh - Qc W = 1200 J - 1000 J = 200 J TH TC QH QC W REFRIGERATOR Pretty Sure Not Quite Sure Just Guessing 04/16/2003 Physics 103, Spring 2005, U. Wisconsin 5 TH TC QH QC Weng HEAT ENGINE TH TC QH QC Wref REFRIGERATOR QH - QC = Weng Eff = Weng / QH Eff = 1 - QC / QH QH - QC = Wref COPrefrig= QC / Wref = QC / (QH - QC) COPht. pump= QH / Weng = QH / (QH - QC) 04/16/2003 Physics 103, Spring 2005, U. Wisconsin 6 Carnot Cycle 04/16/2003 Physics 103, Spring 2005, U. Wisconsin 7 Heat Engine: Carnot Cycle Eff =1 Qc Qh =1 Tc Th No real engine operating between two energy reservoirs can be more efficient than a Carnot engine operating between the same two temperatues. - Sadi Carnot 04/16/2003 Physics 103, Spring 2005, U. Wisconsin 8 Heat Pumps & Refrigerators Heat engines operated backward Work done to transfer heat from cold to hot reservoir Reverse path in the PV diagram compared to Carnot cycle Adiabatic expansion 04/16/2003 Physics 103, Spring 2005, U. Wisconsin 9 Real Engines vs Carnot Engines All real engines are less efficient than the Carnot engine Real engines are irreversible because of friction Real engines are irreversible because they complete cycles in short amounts of time 04/16/2003 Physics 103, Spring 2005, U. Wisconsin 10 Entropy A state variable related to the Second Law of Thermodynamics, the entropy The change in entropy, S, between two equilibrium states is given by the energy, Qr, transferred along the reversible path divided by the absolute temperature, T, of the system in this interval T Q S r= 04/16/2003 Physics 103, Spring 2005, U. Wisconsin 11 Entropy, cont. This applies only to the reversible path, even if the system actually follows an irreversible path To calculate the entropy for an irreversible process, model it as a reversible process When energy is absorbed, Q is positive and entropy increases When energy is expelled, Q is negative and entropy decreases T Q S r= 04/16/2003 Physics 103, Spring 2005, U. Wisconsin 12 More About Entropy Note, the equation defines the change in entropy The entropy of the Universe increases in all natural processes This is another way of expressing the Second Law of Thermodynamics There are processes in which the entropy of a system decreases If the entropy of one system, A, decreases it will be accompanied by the increase of entropy of another system, B. The change in entropy in system B will be greater than that of system A.